This invention relates to remote connection of a tubular string to a subsea installation and particularly for connecting a production riser to a subsea wellhead.
It has been known to run a tubular string by guidelines or through guide rings from floating production vessels or fixed platforms down to seabed installations such as wellheads or mudline casings. A similar operation is conducted in tieback systems after well drilling and well completion. Because of the influence of ocean currents, wind or construction error to the guiding mechanisms on surface platforms, a tubular string frequently will not be aligned directly over a subsea installation.
The current approach to tying back a subsea well is to attempt to remotely stab a tubular connection into or over a subsea receptacle. This subsea connection could be a threaded joint, a "squnch" or stab-type joint, a hydraulically or mechanically actuated connector, or any of a multitude of similar type joints. All that is required of the connection is that it provide a pressure-tight joint that is strong in tension, compression and bending. The current approach works well as long as the tubular string can be positioned vertically over the receptacle prior to make-up. Because of the structural constraints of the platform or production facility, or the effects of the environmental conditions, a vertical connection is sometimes difficult, if not impossible, to obtain.
The most frequently encountered problem is tilted and/or off-center platform. In this case, the platform guide rings constrain the tubular string to remain approximately parallel to the centerline of platform. To make the remote connection, the tubular string must be bent into the appropriate shape to make the transition between the platform centerline and the subsea installation centerline. This bend must take place between the lower platform guide ring and the subsea installation. The tubular string cannot always be pre-bent or it would not pass through the platform guide rings. The only alternative is to bend the lower end of the tubular string remotely.
A tubular string can be bent into a simple arc or an S-shape. To form either shape, a moment must be applied to the tubular string at the lower end. Since the only load inputs available are push, pull, and rotation on the top end of the tubular string, some means must be provided at the bottom end of the tubular string to input a moment. This is typically attempted with a funnel, either on the bottom end of the tubular string or on the top end of the subsea installation. Sometimes an elastomeric flex joint is provided at the lower end of the tubular string to make the tubular string more flexible and thus easier to bend to match the subsea installation centerline. Others have made flex joints that can be remotely bent with hydraulic cylinders or by divers.
When the tubular string is bent into the appropriate shape, the maximum bending moment in the tubular string is located at the two fixed ends. The bending moment is a minimum somewhere near the halfway point between the two fixed ends (the lowest platform guide ring and the subsea installation). Most of the prior art methods of tying back try to bend the tubular string at the lower end where it takes the maximum bending moment to bend the tubular string. In many combinations of platform offset and tilt, the tieback simply cannot be done with the existing systems and the platform must be modified or the operation abandoned.
U.S. Pat. No. 4,433,939 proposes a method of aligning the lower end of a marine conductor to a wellhead to which it is to be connected. After the marine conductor is run into the sea to a location adjacent the wellhead, an articulated bending tool is run from the platform or vessel within the conductor while it is suspended over the wellhead. Hydraulic fluid under pressure is run from the surface facility to the bending tool to induce the desired curvature in the conductor to achieve alignment with the wellhead.
The present invention provides a method whereby the lower portion of a tubular string is first connected to a seabed installation and then loaded at a point away from its lower end. By moving the loading point away from the lower end of the tubular string, the bending deflection for a given input load may be maximized. This allows the operator to tie back with ease and increase the amount that the surface facility can be offset or tilted. Requiring less precision in setting the surface facility results in cost savings during the setting and piling operation.